Coupling systems for interconnecting lines, such as electric lines or fluid conveying lines, must provide reliable service even in adverse operating conditions. For example, the couplings for fluid lines associated with the hydraulic or other systems of aircraft are subjected to vibration, temperature extremes, axial loading and a variety of other difficult operating conditions. Loosening of such couplings in operation could be catastrophic.
Fluid couplings typically comprise a coupling element with a seat and a second coupling element with a mating surface that are joined together by a coupling nut on one of the elements being threaded onto threads on the other element and tightened to force the coupling seat and mating surface together to achieve a fluid tight seal. Thread torque is conventionally used as a measure of whether an appropriate compression of the sealing surfaces to achieve a fluid tight seal has been reached and also as a measure that the threaded surfaces are sufficiently engaged to prevent loosening. Because of the vibration, temperature cycling, axial loading, and other operating conditions of aircraft, threaded connections that have been adequately "torqued" can often loosen after a period of time. In the case of a fluid coupling, even slight loosening will result in the coupling no longer providing the necessary fluid-tight connection. Coupling nuts in aircraft applications have conventionally been adapted to accommodate lockwire as a means of preventing rotation of the torque coupling nut in a loosening direction. Use of lockwire, however, presents problems in maintainability due to the time necessary for installation and accessibility to the coupling.